1. Cross Reference
This application is a continuation-in-part application of co-pending U.S. Ser. No. 07/077,673 filed on July 23, 1983, now abandoned on Dec. 27, 1989.
2. Field of the Invention
This invention relates to a plasma CVD apparatus and particularly, to a low-temperature plasma CVD apparatus which can generate a plasma discharge having excellent stability over an extended period of time. This apparatus is used, for example, for the production of fine particles of metals, alloys and ceramics such as metal oxides and metal nitrides, magnetic fluids composed of fine particles of ferromagnetic metals, alloys, metal oxides or metal nitrides, and colloids containing fine particles of metals, alloys and ceramics.
3. Description of the Prior Art
In conventional plasma CVD apparatuses disclosed, for example, in A. K. Shinha, H. J. Levinstein, T. E. Smith, G. Quitana and S. E. Haszko: J. Electrochem. Soc., 125 (1978), 610, and F. D. Egitto: J. Electrochem. Soc., 127 (1980), 1354, a starting gas blowing port is provided at or near an electrode on the high voltage side, and therefore rises in temperature owing to bombardment of electrons and some ions in a plasma accelerated by an electric field. For example, when one attempts to obtain Fe-N fine particles or an Fe-N magnetic fluid by using Fe(CO).sub.5 and N.sub.2 gas, fine particles of a mixture of Fe, Fe--N and Fe.sub.3 N are always obtained in whatever manner the partial pressure of N.sub.2 gas is controlled. Furthermore, the gas blowing port may be blocked by masses of precipitated Fe, and it is difficult to operate the apparatus continuously for more than 30 minutes. The present Inventors have diligently worked so as to find out a cause of these troubles, and have now discovered that under a reduced pressure of several mmHg in a plasma vapor-phase reaction, Fe(CO).sub.5 readily decomposes thermally at above 100.degree. C. as shown by the following reaction EQU Fe(CO).sub.5 .fwdarw.Fe+5CO
and this reaction occurs in the inside of the gas blowing port by the mechanism described hereinabove.
Accordingly, only thermally stable gases can be used as starting gases for the plasma vapor-phase reaction in the conventional plasma CVD apparatuses. If a thermally unstable starting gas is used, it undergoes a partial chemical reaction by the action of heat inside the nozzle before it is blown into the plasma. Consequently, the reaction product obtained is a mixture of a thermochemical reaction product and a plasma vapor-phase reaction product. The amount of the product yielded decreases, and the product becomes impure. Sometimes, no desired product can be obtained.
This restriction on the type of starting gases that can be used has markedly limited the application of the plasma vapor-phase reaction.
The conventional plasma CVD apparatuses include, for example, (1) a type in which opposing plate electrodes are provided in a fixed box-like plasma reaction vessel equipped with a pressure reduction exhaust opening, and a starting gas and a plasma-generating ambient gas are introduced between these electrodes; and (2) a type in which a high frequency induction coil is wrapped around the outside of a fixed plasma reaction vessel equipped with a pressure reduction exhaust opening, and a starting gas and a plasma-generating ambient gas are introduced into the plasma reaction vessel.
Since these reaction vessels are fixed, they are suitable for forming a film on a fixed substrate placed on a supporting stand provided within the reaction vessel. In the production of fine particles, however, the particles scatter within the reaction vessel and are accumulated everywhere in soot form. They are difficult to capture and the ratio of their recovery is low. Since the temperature within the vessel is non-uniform, the composition of the product accumulated differs depending upon the place of accumulation, or the particle diameter of the fine particles becomes nonuniform. Another disadvantage is that when it is desired to produce magnetic fluids or colloidal products by such an apparatus, it is necessary to provide within the reaction vessel a container for holding a liquid medium and a device for taking the fine particles into the liquid.